Diet therapy and exercise therapy are essential in the treatment of diabetes mellitus. When these therapies do not sufficiently control the conditions of patients, insulin or an oral antidiabetic agent is additionally used for the treatment of diabetes. At the present, there have been used as an antidiabetic agent biguanide compounds, sulfonylurea compounds, insulin resistance improving agents and α-glucosidase inhibitors. However, these antidiabetic agents have various side effects. For example, biguanide compounds cause lactic acidosis, sulfonylurea compounds cause significant hypoglycemia, insulin resistance improving agents cause edema and heart failure, and α-glucosidase inhibitors cause abdominal bloating and diarrhea. Under such circumstances, it has been desired to develop novel drugs for treatment of diabetes mellitus having no such side effects.
Recently, it has been reported that hyperglycemia participates in the onset and progressive impairment of diabetes mellitus, i.e., glucose toxicity theory. Namely, chronic hyperglycemia leads to decrease of insulin secretion and further to decrease of insulin sensitivity, and as a result, the blood glucose concentration is increased so that diabetes mellitus is self-exacerbated [Unger, R. H., et al., “Hyperglycemia as an inducer as well as a consequence of impaired islet cell function and insulin resistance: implication for the management of diabetes”, Diabetoloaia, 1985, vol. 28, pp 119-121; Rossetti, L., et al., “Glucose Toxicity”, Diabetes Care, 1990, vol. 13, no. 6, pp 610-630]. Therefore, by treating hyperglycemia, the aforementioned self-exacerbating cycle is interrupted so that the prophylaxis or treatment of diabetes mellitus is made possible.
As one of the methods for treating hyperglycemia, it is considered to excrete an excess amount of glucose directly into urine so that the blood glucose concentration is normalized. For example, by inhibiting sodium-dependent glucose transporter being present at the proximal convoluted tubule of kidney, the re-absorption of glucose at the kidney is inhibited, by which the excretion of glucose into urine is promoted so that the blood glucose level is decreased. In fact, it is confirmed that by continuous subcutaneous administration of phlorizin having SGLT inhibitory activity to diabetic animal models, hyperglycemia is normalized and the blood glucose level thereof can be kept normal for a long time so that the insulin secretion and insulin resistance are improved [Rossetti, L., et al., “Correction of Hyperglycemia with Phlorizin Normalizes Tissue sensitivity to Insulin in Diabetic Rats”, J. Clin. Invest. 1987, vol. 79, pp 1510-1515; Rossetti, L., et al., “Effect of Chronic Hyperglycemia on in Vivo Insulin Secretion in Partially Pancreatectomized Rats”, J. Clin Invest. 1987, vol. 80, pp 1037-1044; Kahn, B. B., et al., “Normalization of blood glucose in diabetic rats with phlorizin treatment reverses insulin-resistant glucose transport in adipose cells without restoring glucose transporter gene expression”, J. Clin. Invest., 1991, vol. 87, pp 561-570]
In addition, by treating diabetic animal models with SGLT inhibitory agents for a long time, insulin secretion response and insulin sensitivity of the animals are improved without incurring any adverse affects on the kidney or imbalance in blood levels of electrolytes, and as a result, the onset and progress of diabetic nephropathy and diabetic neuropathy are prevented [Tsujihara, K., et al., “Na+-Glucose Cotransporter (SGLT) Inhibitors as Antidiabetic Agents. 4. Synthesis and Pharmacological Properties of 4′-Dehydroxyphlorizin Derivatives Substituted on the B Ring”, J. Med. Chem. 1999, vol. 42, pp 5311-5324; Arakawa, K., et al., “Improved diabetic syndrome in C57BL/Ks-db/db mice by oral administration of the Na+-glucose cotransporter inhibitor T-1095, Br. J. Pharmacol., 2001, vol. 132, pp 578-586; Ueta, K., et al., “Long-term treatment with the Na+-glucose cotransporter inhibitor T-1095 causes sustained improvement in hyperglycemia and prevents diabetic neuropathy in Goto-kakizaki Rats”, Life Sciences, 2005, vol. 76, pp 2655-2668].
From the above, SGLT inhibitors may be expected to improve insulin secretion and insulin resistance by decreasing the blood glucose level in diabetic patients and further prevent the onset and progress of diabetes mellitus and diabetic complications.